An electrophotographic printing machine forms images by an electrophotography as described below. First, a photosensitive layer formed on the surface of a photoreceptor is uniformly charged with a single polarity. Then, an electrostatic latent image is formed on the surface of the photoreceptor through the process of exposure. Further, toner is made to adhere to the electrostatic latent image, thereby forming images.
In the electrophotographic printing machine thus described, conventionally a corona discharger has been used as a charger for uniformly charging with a single polarity the photosensitive layer formed on the surface of the photoreceptor. However, in the case of adopting the corona discharger, an application of high voltage is required for a wire electrode when charging the photosensitive layer. This presents the problem since a power supply for applying voltage to the wire electrode becomes larger in size. Moreover, with the corona discharger, an erosion of the components of the machine and the deterioration of the surface of the photoreceptor are likely to occur due to ozone produced in the process of corona-discharging the photosensitive layer. This results in the problem by making unclear and fuzzy the images, or by adversely affecting the human body, etc.
In order to counteract the above-mentioned problems, a contact charger has been recently proposed for a charger which does not require a large power source nor produces ozone. Such contact charger is provided with an electrically conductive contacting component in contact with the photosensitive layer formed on the surface of the photoreceptor. With the contact charger, the photosensitive layer formed on the surface of the photoreceptor is charged through the contacting component by applying thereto DC voltage through the contacting component. With reference to FIG. 8, the following will describe the electrophotographic printing machine provided with the contact charger through the case of adopting a charge roller for a contacting component among those roller-shaped, blade-shaped, and brush-shaped.
A contact charger 100 is provided with a charge roller 101 and a power supply 102 for applying DC voltage to the charge roller 101. The charge roller 101 is arranged such that an electrically conductive elastic layer is formed on a surface of an electrically conductive cylindrical roller base 101b. The charge roller 101 is freely rotative around an axis parallel to a rotation axis of a photoreceptor 103. Further, the charge roller 101 is in contact with the surface of the photoreceptor 103 with a predetermined nip width. The charge roller 101 is rotated in the direction of D in conjunction with the rotation of the photoreceptor 103 in the direction of C in the figure.
The power supply 102 being connected to the roller base of the charge roller 101 applies DC voltage to the surface of the photoreceptor 103 through the charge roller 101.
Here, the photoreceptor 103 is designed so as to have the photosensitive layer 103a on the surface of a drum base 103b having a property as an insulator when it is not exposed, on the other hand, having an electric property which makes the exposed portion thereof electrically conductive when it is exposed. The drum base 103b is made of an electrically conductive material and connected to ground so that a charge on the exposed portion of the photosensitive layer 103a is released to ground.
With the above arrangement, the charge is moved onto the photosensitive layer 103a of the photoreceptor 103 through the charge roller 101 whereon DC voltage has been applied from the power supply 102, thereby uniformly charging the photosensitive layer 103a.
However, with the above contact charger 100, since the surface of the photoreceptor 103 is in contact with the charge roller 101, a chemical change due to an erosion, etc., is likely to occur on the surface of the photoreceptor 103 which may create a defect such as a pin hole on the photosensitive layer 103a of the photoreceptor 103. Moreover, when dusts or contaminants are inserted into the contacting portion between the surface of the photoreceptor 103 and the charge roller 101, a defect is likely to occur on the photosensitive layer 103a of the photoreceptor 103.
With the electrophotographic printing machine employing the contract charger 100, the following problems may arise when a defect exists such as a pin hole on the photosensitive layer 103a formed on the surface of the photoreceptor 103.
Namely, at the defective portion of the photosensitive layer 103a, electrically conductive drum base 103b is exposed. Therefore, there will be a continuity between the surface of the charge roller 101 and the drum base 103b when the charge roller 101 is in contact with the defective portion of the photosensitive layer 103a. This means that excessive current flows between the charge roller 101 and the base of the photoreceptor 103, and thus voltage of the charge roller 101 suddenly drops. As a result, an improper charge (drop in the charged potential) occurs in an axis direction at the defective portion on the photosensitive layer 103a formed on the surface of the photoreceptor 103, thereby presenting the problem of an irregular image.
Moreover, heat is generated by the excessive current flowing across the charge roller 101 and the drum base 103b of the photoreceptor 103. If this occurs, the photosensitive layer 103 is more quickly deteriorated. Further, if the image forming process is continued, the contact charger 100 may break down, and the electrophotographic printing machine cannot be used.